Quasi-distributed optical fiber sensors are known from the prior art, for example for the underwater detection of acoustic waves (coastline surveillance, seismic activity monitoring, etc.) or the detection of pressure variation in the air.
These sensors are widely used for their compactness and their sensitivity.
The most sensitive are based on active (laser) Bragg gratings coupled to a mechanical transducer which converts the radial pressure of the acoustic wave into elongation of the fiber, leading to a modulation of the optical wavelength at the fiber output.
For example, the distributed feedback fiber laser (DFB-FL) sensor as described in the publication “All optical array for underwater surveillance”, European Conference Workshop on Optical Fiber Sensors EWOFS 2013, Krakow, Poland, Proceeding SPIE Vol. 879487940N-1, comprises an optical fiber in which a plurality of Bragg gratings have been registered, producing a plurality of laser cavities suitable for each lasering on a wavelength λi, each cavity forming a sensor.
The principle of operation of the distributed feedback fiber laser sensor is based on the axial deformation of the laser cavity, which modulates the frequency of the laser. The acoustic signal axially deforms the sensitive center of the laser cavity. The deformation causes the optical phase of the cavity, and therefore the resonant frequency thereof, and its emission optical frequency, to be modulated.
An acoustic wave located in proximity to the fiber is transformed into elongation, which induces a modulation of the emission wavelength of the laser situated in the fiber and in proximity to the acoustic wave. The light from the lasers is recovered at the fiber output and coupled to an interferometer, then injected into a wavelength demultiplexer to identify which laser has undergone the change of wavelength, and determine therefrom the direction of arrival of the acoustic wave. These sensors are capable of detecting the picostrain, i.e. relative variations of ΔL/L˜10−12.
These sensors and systems based on Bragg gratings are intrinsically sensitive to the variations of the environment, typically temperature and static pressure, which places strong constraints on their design. The mechanical structure in which the cavity is housed is configured so that, when the temperature varies, it undergoes a deformation leading to a variation of the length of the cavity which neutralizes that induced by the static pressure variations. The mechanical structure also has a number of orifices for equalizing the static pressures, making it possible to avoid a bulky hydrostatic filter.
Furthermore, accurate interferometric measurements have to be performed to transform the modulation of the optical frequency into a phase modulation. The technique used is, for example, of the delayed self-homodyne type and requires a great length of delay line (optical fiber capable of introducing phase noise on the signal) and at least one frequency translation (acousto-optical or electro-optical element inducing losses). These measurements further require a feedback loop to neutralize the slow variations of the length of the fiber delay line and keep the interferometer in a linear zone of operation (quadrature) that is stable and of maximum sensitivity.
Thus, the use of an interferometer constitutes a limitation on the practical implementation of the systems, particularly when the detection has to be executed in a fluctuating environment.
These sensors are also expensive, because they consist of laser cavities that are costly to fabricate and to implement.